The present invention relates to a gas combustion plant comprising a combustion chamber, a burner for introducing the gas to the combustion chamber and a tubular heat exchanger for preheating the gas to be combusted, by means of the hot combustion gases.
Since long waste gases having a high content of combustible components have been burned as a torch or in firing plants in order to decompose toxic, explosive or smelling components into relatively harmless components like CO.sub.2, H.sub.2 O and SO.sub.2. Such combustion is nowadays increasingly used also when the content of combustible components in the gas is so low that the gas is not self-combustible and therefore cannot be brought to the temperature (500.degree. to 850.degree. C) necessary for obtaining a sufficient oxidation rate. The gases must also be kept at said temperature for a certain period for the desired degree of oxidation to be obtained. Finally, a good turbulence of the burning gases is important for obtaining a good result.
Known plants for such termic gas purification may comprise a combustion chamber and a burner for introduction of a fuel, such as oil, propane or the like in addition to the gas to be cleaned. Such plants may be used when the gas to be cleaned has a high temperature or a high content of combustible components. In principle the plant is similar to the afterburner chambers used in destruction furnaces for waste and the like. When the gas to be cleaned has a low temperature it may be advantageous to preheat the gas, and this may be effected by heat exchange with the hot exhaust gases. Thus, plants are known having a heat exchanger arranged after the combustion chamber, the hot combustion gases being passed through straight tubes in the heat exchanger, whereas the gas to be burned is passed through the heat exchanger transversely to the tubes in order to be heated and then delivered to the combustion chamber at an inlet end opposite to the outlet end for the exhaust gases. In order to obtain good mixing (turbulence) of burnt and unburnt gas, the combustion chamber may have the form of a vortex or whirl chamber or the gas inlet may be provided with guide blades.
These plants work well, but may require relatively high temperatures, depending on the design of the combustion chamber, in order to obtain a sufficient oxidation of the gas. Especially in the oxidation of CO, which is a relatively heavily oxidizable gas, it may be necessary to maintain a high temperature if the residence time in the combustion chamber is not sufficiently long and the turbulence conditions are not sufficiently good. Especially the transitional portion between the combustion chamber and the heat exchanger is exposed. If a hole should be burned in the heat exchanger in this position a short-circuiting occurs between cold and hot gas, meaning that cold, unpurified gas will be carried along with the hot exhaust gases and not be purified. The flow path of the gas to be heated through the heat exchange involves a tendency for any entrained dust to deposit on the outside of the tubes of the heat exchanger, thereby reducing the heat exchange and increasing the temperature in the heat exchanger tubes with an increasing risk of thermal damages thereof.